Question

Calculate the percentage composition of iron in ferric oxide, $F{{e}_{2}}{{O}_{3}}$ (atomic mass of Fe = 56)
A. 70%
B. 56%
C. 87%
D. 35%

Answer 1

We have to follow the below formula to calculate the percentage of any ion present in the combined chemical form.
$\text{ percentage of metal in the chemical = }\dfrac{\text{total mass of the metal}}{\text{total mass of the compound}}\times \text{ 100}$

Complete answer:
Molecular formula of ferric oxide is $F{{e}_{2}}{{O}_{3}}$.
Initially we have to calculate the total mass of the ferric oxide.
Ferric oxide has two iron atoms and three oxygen atoms.
Molecular weight of oxygen = 16
Molecular weight of iron = 56

& \text{Total mass of oxygen=3 }\\!\\!\times\\!\\!\text{ 16} \\\ & \text{ =48 grams of oxygen} \\\ & \text{Total mass of iron=2 }\\!\\!\times\\!\\!\text{ 56} \\\ & \text{ =112 grams of iron} \\\ \end{aligned}$$ Adding the mass of total oxygen and total iron gives the molecular weight of the ferric oxide. Total mass of ferric oxide = 48+ 112 =160 grams Percentage of an atom in the chemical $$\begin{aligned} & \text{ percentage of iron in the chemical = }\dfrac{\text{total mass of the iron}}{\text{total mass of the ferric oxide}}\text{ }\times \text{ 100} \\\ & \text{=}\dfrac{\text{112}}{\text{160}}\text{ }\times \text{ 100} \\\ & \text{=70 }\text{% } \\\ \end{aligned}$$ The percentage of iron in ferric oxide is 70%. **So, the correct option is A.** **Additional information:** Both ferrous oxide and ferric oxide are in-organic compounds. Ferric oxide is a more stable form of iron than the ferrous oxide. Ferric oxide $F{{e}_{2}}{{O}_{3}}$ is a reddish-brown powder. Ferrous oxide $FeO$ is a black-colored powder. **Note:** Don’t be confused with the words ferrous oxide and ferric oxide: The molecular formula of Ferrous oxide- $FeO$ The molecular formula of Ferric oxide- $F{{e}_{2}}{{O}_{3}}$ In ferrous oxide $FeO$ the oxidation state of iron is +2. In ferric oxide $F{{e}_{2}}{{O}_{3}}$ the oxidation state of iron is +3.